1. Field of the Invention
The invention is directed to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One fuel injection system of the type with which this invention is concerned, known from European Patent Disclosure EP 0 957 261 A1, has one high-pressure fuel pump and one fuel injection valve, communicating with it, for each cylinder of the engine. The high-pressure fuel pump has a pump piston, driven in a reciprocating motion by the engine, and this piston defines a pump work chamber that can be made to communicate with a pressure chamber of the fuel injection valve, which valve has an injection valve member by which at least one injection opening is controlled and which is movable by the pressure prevailing in the pressure chamber, counter to a closing force, in an opening direction to uncover the at least one injection opening. A first electrically actuated control valve is provided, which can be switched back and forth between two switching positions and by which a communication of the pump work chamber with a relief chamber is controlled. A second electrically actuated control valve is also provided, by which the pressure prevailing in a control pressure chamber is controlled, by which pressure the injection valve member is urged in the closing direction. The control pressure chamber has a communication with the pump work chamber, and by means of the second control valve, a communication of the control pressure chamber with a relief chamber is controlled. A disadvantage of this known fuel injection system is that the course of the fuel injection, or in other words the injected fuel quantity and the pressure at which the fuel injection is effected, can vary to only a limited extent during an injection cycle. In particular, in an injection cycle with a preinjection and a subsequent main injection, the pressure at which the main injection begins, and the spacing between the main injection and the preinjection are coupled with one another and are not freely variable. If the main injection is meant to begin at a slight pressure, then the spacing from the preinjection is only slight, and if the main injection is meant to begin at a high pressure, then the spacing from the preinjection is long.
The fuel injection system of the invention has the advantage over the prior art that by means of the first control valve, in its first switching position, an elevated pressure can be maintained in the pressure chamber and in the control pressure chamber, even when the pump work chamber is in communication with the relief chamber, so that regardless of a relief of the pump work chamber by means of the second control valve, a fuel injection can be controlled, particularly for a preinjection and/or a postinjection. The pressure buildup for a main injection can be controlled by the first control valve, and the instant at which the main injection begins can be controlled by the second control valve. This makes a decoupling possible between the pressure at which the main injection begins and the spacing from a preceding preinjection.
Other advantageous features and refinements of the fuel injection system of the invention are disclosed. One embodiment enables a simultaneous relief of the pump work chamber, the pressure chamber and the control pressure chamber. Another embodiment makes a control of the pressure in the control pressure chamber possible in a simple way, while another enables adjusting the fuel inflow into the control pressure chamber and the fuel outflow from the control pressure chamber. A further embodiment makes it possible to operate the engine with low noise and pollutant emissions while another, in a simple way, enables adjusting the fuel quantity for the preinjection by means of the length of time for which the first control valve is closed. Further embodiments make it possible in a simple and purely mechanical way to adjust the fuel quantity for the preinjection, enables a postinjection, without fuel having to be pumped by the pump piston during the postinjection, makes it possible in a simple way to perform a preinjection, and enables a relief of the pressure chamber and of the control pressure chamber.